Fix undefined method `[]' for nil:NilClass from rspec -e.

`$options` was only set in `CLI#run` so running only non-CLI spec examples
could fail because `$options` was nil when the array index operator was
called on it.

This solution is different from [my previous attempt](https://github.com/bbatsov/rubocop/pull/105/files#diff-2). The problem only occurs when running specs so now the solution is under `specs/`.